As shown in FIG. 1, a conventional crank assembly 1 for a bicycle comprises a first crank arm 11, a sprocket 12 fixed to the first crank arm 11, a crank shaft 13 one end of which is connected to the first crank arm 11, a second crank arm 14 connected to another end of the crank shaft 13. A plurality of splines are formed on an outer peripheral surface at two ends of the crank shaft 13. One end, connected to the crank shaft 13, of each of the first crank arm 11 and the second crank arm 14 is formed with a splined bore 15. Two ends of the crank shaft 13 are respectively received in the splined bores 15 of the first crank arm 11 and the second crank arm 14 such that the splines of the crank shaft 13 engage with the splined bores 15 of the first crank arm 11 and the second crank arm 14.
Due to the engagement, the second crank arm 14 can rotate the crank shaft 13 via the splines on one end of the crank shaft 13 and the crank shaft 13 can rotate the sprocket 12 via the splines on another end of the crank shaft 13 when a rider of the bicycle applies a force to another end, to be connected to a pedal (not shown), of the second crank arm 14.
However, a lot of problems exist in the conventional crank assembly 1 aforementioned in which the crank shaft 13 and sprocket 12 is rotated by splines.
First, the dimensions of the splines are small and strength thereof is weak. Therefore, after the bicycle has been used for a period of time, the splines will deform or be worn such that free play occurs between the crank shaft 13 and the first crank arm 11 and the second crank arm 14. Such free play will result in incomplete transmission. Especially, a mountain bicycle encounters much obstruction when it runs on a rugged and rough terrain. At this time, the splines which are responsible for rotating the sprocket 12 and a chain will receive a large force. The large external force may cause the splines to suddenly break and lose its function of transmission. Such an accident may bring about the rider tumbling over and place the rider in a risk. In brief, the strength of the splines of the conventional crank assembly 1 is too weak so that the force can be transmitted by the splines is not large enough.
Next, both the first crank arm 11 and the second crank arm 14 are used to rotate the crank shaft 13. The length of the force arms on the first crank arm 11 and the second crank arm 14 is limited by the distance from the pedal to the crank shaft 13. The force arms on the first crank arm 11 and the second crank arm 14 cannot be increased to be longer than such a distance. In other words, the labor-saving effect cannot be uprisen.
Moreover, since one end of each of the first crank arm 11 and the second crank arm 14 is formed with a splined bore, material which surrounds the splined bore becomes thin and the strength of the one ends of the first crank arm 11 and the second crank arm 14 is decreased. Hence, the material which surrounds the splined bore is easy to crack when external forces are applied to the first crank arm 11 and the second crank arm 14.
Besides, conventional crank assembly 1 comprises too many parts, which increases the cost.